Terry Stewart's (Tezza's) Projects and Articles (Blog)

My past de-yellowings have used a paste
made from a common food thickener called Arrowroot. I used Arrowroot rather
than Xanthan gum (as previous people had done), as I couldn't easily find
the latter. Arrowroot worked well on my cases to date (see other blog
entries). However, other folk who had compared the two found the Arrowroot
dried
quicker than Xanthan gum/glycerine and was also less transparent.

This test initially sought simply to compare
the Arrowroot and Xanthan gum/glycerine paste. However, eventual results
also strengthens the theory that allowing the paste to dry for long periods
under the hot sun can lead to uneven discolouration and a bleaching of
the case (as opposed to simply reverting to the original colour).

The treatments

Two pastes were made up. My own recipe
using Arrowroot and Lorne's Xanthan gum/glycerine variation from the original
recipe. During the treatment period these were applied at the same time
to a side of the test case (Xanthan gum/glycerine left side, Arrowroot,
right side).

A mahogany coloured Commodore 64 case
was used for the trial. A band of tape down the middle was used to separate
the two treatments.

Treatment was over four days inside using
the New Zealand sun as a UV source and one day in a humid chamber sealed
with cellophane under a UV lamp. UV levels were very inconsistent over
the first four days due to cloudy weather. For the days outdoors, the
paste was reapplied every two hours or so, with the exception of the second
day, where the case was left in the sun for a whole day with just one
application (as I had run out of ingredients).

Photo 1. Xantan gum/glycerine on left, Arrowroot on
right.

The Arrowroot paste did appear to dry
faster than the Xanthan gum/glycerine mix, leaving a kind of flaky layer
on the plastic. The Xanthan gum/glycerine mix was relatively clear, and
stayed moist longer.

The following table details the treatment.

Day

Conditions

Temperature (approx)

Applications

1

Cloudy, dim, weather. Light drizzle
sometimes

20 deg C

5

2

Sunny, a breeze. Good drying conditions

22 deg C

1*

3

Partly cloudy. Sun visible 40% of the
time

20 deg C

5

4

Sunny, a breeze. Good dry conditions

25 deg C

5

5

Under a UV lamp in a humid chamber
(paste remained moist all day, no further applications required)

18 deg C

1

* Early in the day

Results

Photos 2, 3 and 4 show the case at the
beginning of the trial, after day two and at the conclusion after day
five.

Photo 2. Case before trial

Photo 3. Case after day 2. I believe this is where
the damage occurred. A hot day, and paste was allowed to dry out. Note
the bleached areas.

Photo 4. Case at the conclusion of
the trial

Both pastes caused the case to revert
to it's natural colour. However, with both the Xanthan gum/glycerine and
Arrowroot, the result was not as smooth and uniform as the original grey/beige
colour, which could be found on the inside of the case

Photo 5. Original colour of the case

There were areas where the case had whitened,
beyond it's natural colour. Although subtle, some of the textured areas
had a kind of "white bloom" when examined carefully.

Photo 6. Bleaching symptom from paste drying out in
the hot sun

This occurred using both pastes, but seemed
to be a little more pronounced with the Arrowroot. Wetting the plastic
would make this phenomena disappear, but it would return on drying. It
appears to be a similar symptom to that mention in item 1 under "Problems
and Pitfalls" in The
RetroBright Wiki.

Discussion

Both pastes de-coloured the case successfully.
Arrowroot certainly dried faster. It would be interesting to know if the
addition of glycerine would slow this.

However, with both pastes the de-colourisation
was not uniform and even. Some patches are lighter than others. The light
patches appear lighter than the original colour and may be a result of
the peroxide attacking the plastic itself.

My thoughts are the damage may have occurred
on day 2, when the case was left for a long period with just one application.
These lighter patches could already be seen after that day (see photo
3 above). It's possible that under high drying conditions, the local concentration
of peroxide on the surface in some areas became very high, and bleached
the plastic.

Assuming the theory as to why the bleaching
occurs is correct, it would be wise not to let the cases dry out under
strong sunlight. On a white case, bleaching might not be visible but a
UV bulb and no added heat may be the prudent way to go with a valued case
whose natural colour is anything other than white.

Summary

Xanthan gum/glycerine and Arrowroot both de-yellow
cases to a similar extent. However, Arrowroot dries out faster. This
does not seem to effect its rate of de-yellowing but it can give a
slightly (but not significantly) patchier result than Xanthan gum/glycerine.

Both pastes may cause a slight bleaching some plastics
under fast drying conditions with heat (e.g. strong sunlight)

A UV light, no heat and repasting when the paste
appears to be drying out would be prudent for a valuable case.

I hope this experience adds to the knowledge
base regarding this process.